Japan's Mitsubishi Heavy Industries, Ltd. (MHI) and Nippon Steel Corporation have jointly developed a technology to use higher tensile strength steel (HTSS) with yield stress of 47 kgf/mm2 for the longitudinal strength member of supersized containerships.

The longitudinal strength member is the most important part of a containership's hull.

New containerships are increasing in size and the steel plates used for them are also becoming thicker.

However, says MHI, when "plates become thicker, toughness tends to decline. Use of the new HTSS, which has successfully achieved toughness in addition to increased strength and reduced thickness, will not only contribute to improvements in weight reduction and fuel efficiency, but also increase the reliability of the ship's hull."

When toughness is high, the possibility of crack initiation will be reduced and resistance to crack propagation increases.

The first containership to incorporate the new steel will be built at MHI's Nagasaki Shipyard and Machinery Works while the HTSS steel plate will be produced at Nippon Steel's Oita Works.

MHI says the 47 kgf/mm2 HTSS is the world's highest strength steel plate for the hulls of commercial ships. In addition to improved hull safety with higher toughness steel, the reduced volume of steel and resultant lighter ship weight will allow greater deadweight.

Currently, the highest strength steel plate being used for commercial ships is 40 kgf/mm2 HTSS, introduced fifteen years ago.

Nippon Steel has developed the new HTSS by applying its Thermo-Mechanical Control Process (TMCP) technology, a production process that concurrently enhances strength, toughness and weldability of steel through hot rolling and online water-cooling.

The company has verified the outstanding safety of the steel by using a test facility with giant tensile capacity of 8,000 tons.

Nippon Kaiji Kyokai (Class NK) also participated in the development.

MHI designed the hull structure, leveraging characteristics of the steel and realizing increased safety for the hull structure through optimization such as reduced plate thickness, and steel and welding arrangement.

In general, steel weldability deteriorates in relation to increasing strength. One of the features of the new HTSS is claimed to be excellent weldability--equivalent to 40 kgf/mm2 HTSS.

MHI has established the most suitable welding method for the new HTSS by conducting various welding tests, including two-electrode VEGA (Vibratory Electro Gas Arc) welding, jointly developed by MHI, Nippon Steel Corporation and Nippon Steel & Sumikin Welding Co., Ltd.

"By applying this welding method," says MHI, "it is evident that the resulting product is superior in strength, toughness and the quality of welded parts over existing HTSS."

With the introduction of large-size containerships that combine highly reliable 47 kgf/mm2 HTSS and MHI's special design and construction methods, the company says it is responding to the increasing needs of customers for higher transportation efficiency and reduced environmental load through improved fuel efficiency and enhanced safety of ship's hull.